The present invention relates to double-beam spectrophotometers of the type wherein a beam from a suitable radiation source is directed alternately into a sample optical channel and a reference optical channel and is then re-directed alternately from one and other channel along a common optical path through a monochromator and onto a detector capable of generating an electrical output proportional to the intensity of the intercepted radiation, the dividing and recombining of the said beam performed by a constant-rate beam-switching arrangement such that the detector output comprises a sample signal component and a reference signal component that may be compared by generating the ratio between the two. In other words, the present invention relates to double-beam time-sharing ratio-recording spectrophotometers but not to spectrophotometers in which the beams are coincident in time.
The term "recording" in the phrase "ratio-recording", which has a well recognized meaning in the art, is here intended to refer to any convenient presentation of said ratio leading to the production of a permanent record, e.g. chart recording, or a transient display, e.g. digital read out.
The object of any double-beam arrangement is to remove from the output signal of the spectrophotometer the effect of spurious absorption that is common to both optical channels, e.g. absorption by atmospheric water vapour. The said arrangment operates, therefore, on the common mode rejection principle.
In spectrophotometers of the type to which the present invention applies, the beam of radiation exists in one optical channel for one half of the time and in the other channel for the other half. This means that the detector responds to the two channels sequentially or, in other words, the measurement of sample channel radiation is not simultaneous with the measurement of reference channel radiation. It clearly follows that if an absorption change takes place in one optical channel that has no exact counterpart in the other channel a common mode rejection failure must inevitably result. In the art this form of failure is referred to as "uncompensation".
In so far as absorption is, of course, wavelength dependent, uncompensation must arise in any time-sharing double-beam ratio-recording spectrophotometer used in the scanning mode, since the radiation emerging from the two channels is not measured at the same wavelength and consequently the ratio between the two measurements will not eliminate, for example, the effect of water vapour absorption. Uncompensation can be particularly troublesome in infrared spectrophotometers having extended optical path lengths.